Metal contact structure for solar cell and method of manufacture

ABSTRACT

In a solar cell having p doped regions and n doped regions alternately formed in a surface of a semiconductor wafer in offset levels through use of masking and etching techniques, metal contacts are made to the p regions and n regions by first forming a base layer contacting the p doped regions and n doped regions which functions as an antireflection layer, and then forming a barrier layer, such as titanium tungsten or chromium, and a conductive layer such as copper over the barrier layer. Preferably the conductive layer is a plating layer and the thickness thereof can be increased by plating.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to solar cells, and moreparticularly the invention relates to metal contact structures for usein solar cells.

[0002] The use of photovoltaic cells for the direct conversion of solarradiation into electrical energy is well known, see Swanson, U.S. Pat.No. 4,234,352 for example. Briefly, the photovoltaic cell comprises asubstrate of semiconductive material having a p-n junction definedtherein. In the planar silicon cell the p-n junction is formed near asurface of the substrate which receives impinging radiation. Radiatedphotons create mobile carriers (holes and electrons) and the substratewhich can be directed to an electrical circuit outside of the cell. Onlyphotons having at least a minimum energy level (e.g., 1.1 electron voltfor silicon) can generate an electron-hole pair in the semiconductorpair. Photons having less energy are either not absorbed or are absorbedas heat, and the excess energy of photons having more than 1.1 electronvolt energy (e.g., photons have a wavelength of 1.1 μm and less) createheat. These and other losses limit the efficiency of photovoltaic cellsin directly converting solar energy to electricity to less than 30%.

[0003] Solar cells with interdigitated contacts of opposite polarity onthe back surface of the cell are known and have numerous advantages overconventional solar cells with front side metal grids and blanket or gridmetallized backside contacts, including improved photo-generation due toelimination of front grid shading, much reduced grid series resistance,and improved “blue” photo-response since heavy front surface doping isnot required to minimize front contact resistance and since there are nofront contacts. In addition to the performance advantages, theback/contact cell structure allows simplified module assembly due tocoplanar contacts. See Swanson U.S. Pat. No. 4,927,770 for example.

[0004] The present invention is directed to an improved metal contactstructure which is especially applicable to solar cells.

BRIEF SUMMARY OF THE INVENTION

[0005] In accordance with the invention, a solar cell has a metalcontact structure including a first metal layer in contact with thesemiconductor substrate which can also function as an infraredreflector. A diffusion barrier metal layer covers the first metal layerand provides a base for plating additional metal.

[0006] In a preferred embodiment, a silicon cell having a first majorsurface for receiving solar radiation has an opposing or backsidesurface in which p-doped and n-doped regions are formed in a spacedparallel arrangement. Interdigitated metal contacts and grid linesrespectively contact the p and n doped regions.

[0007] In forming the interdigitated metal contacts to the p and nregions, arrays of small contact openings are fabricated in the siliconoxide layer by using a patterned etch resist and chemical etching. Aseed layer metal stack is then sputtered on the back side of the cell.The first metal in the stack provides ohmic contact to the siliconthrough the contact openings in the oxide and acts as an infraredreflector. A second metal layer acts as a diffusion barrier and adhesionlayer. A top metal layer then forms a base to initiate plating. Apatterned plating resist is then applied over the seed layer, and metalis plated on the cell to build up thickness for the metal grid lines.Finally, the plating resist is stripped, and the metal layer between thegrid lines is removed by chemical etching.

[0008] The invention and objects and features thereof will be morereadily apparent from the following detailed description and appendedclaims when taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a perspective view illustrating the back side of afinished solar cell with metal contacts fabricated in accordance withone embodiment of the invention.

[0010]FIGS. 2-8 are side views in section illustrating steps infabricating a metal contact structure for a solar cell in accordancewith one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011]FIG. 1 is a perspective view of a solar cell in which metalcontacts in accordance with the invention are especially applicable. Inthis embodiment, the cell is preferably manufactured in a singlecrystalline silicon substrate having a (100) crystalline orientation orin a multi-crystalline silicon substrate with minority carrier lifetimegreater than 200 micro-seconds.

[0012] In this embodiment, a front surface of the solar cell has atextured surface 54. An antireflection coating can be applied to assistin the coupling of light energy into the solar cell and improveefficiency. On a back surface, metal contacts 50, 52 in accordance withthe invention contact p doped regions and n doped regions respectively,in spaced layers of the back surface. The contacts are respectivelyconnected with grid lines 51, 53 in a grid pattern. The line size isexaggerated in the drawing. The fabrication of the solar cell usesconventional semiconductor processing, including the use of backsidediffusions, and the texturing of the front surface. Since these processsteps form no part of the present invention, further description of thesemiconductor processing is not provided.

[0013] Consider now the metal contacts 50, 52 and fabrication thereof inaccordance with the invention. A preferred embodiment will be describedwith reference to the side views in section of wafer 10 shown in FIGS.2-8.

[0014] In FIG. 2, wafer 10 has the textured front surface including adoped layer 28, a silicon oxide layer 30, and an antireflection coating(ARC) 32 such as SiN or TiO₂ made from earlier processing steps. Theback surface has p+ regions 12 and n+ regions 18 in spaced levels withan overlying silicon oxide layer 14. The p+ and n+ regions can be madein accordance with the teachings of Sinton U.S. Pat. No. 5,053,083.

[0015] As shown in FIG. 3, a patterned etch resist 40 is applied overthe back side silicon oxide 14. Resist 40 is then either thermal or UVcured. Depending on the ARC material, a patterned etch resist may beapplied over the front of the solar cell to protect the ARC fromsubsequent etching. In FIG. 4, arrays of small contact openings 42 arechemically etched in the silicon oxide over both the p and n regions 12,18, then the etch resist 40 is stripped using a caustic solution. Thetotal contact area as a fraction of the entire back side is typicallyless than 5%. Reducing the metal to semiconductor contact area greatlyreduces photo-generated carrier recombination at the back surface of thesolar cell, and hence increases cell efficiency.

[0016] Alternatively, the contact mask and contact oxide etch can beeliminated from the process and contact openings can be formed in theoxide layer by other methods, such as laser ablation of oxide, or directprinting of chemical pastes that etch the oxide.

[0017] In FIG. 5, a thin (approximately 400 nm) 3-layer seed metal stack44 is sputtered or evaporated onto the solar cell for contacts to p+region 12 and n+ region 18. The first layer of the stack, aluminum inthe preferred embodiment, makes ohmic contact to the semiconductormaterial and acts as a back surface reflector. In thin silicon solarcells, weakly absorbed infrared radiation passes through the thicknessof silicon and is often lost by absorption in backside metallization. Inone embodiment, the seed layer covers mostly silicon oxide, except insmall contact openings where it contacts the silicon. The metallizedsilicon oxide stack is designed to be an excellent infrared reflector,reflecting light back into the cell and effectively multiplying theabsorption path length. The front surface texture in combination withthe back surface reflector can increase the optical path length to morethan twenty times the wafer thickness. This design feature leads tohigher photo-generated current in the solar cell.

[0018] A second layer, titanium-10%/tungsten-90% (TiW) in the preferredembodiment acts as a diffusion barrier to metals and other impurities. Athird layer, copper (Cu) in the preferred embodiment, is used to providea base or strike layer for initiating electroplating of metal.Alternatively, chromium (Cr) or nickel can be used as the barrier layerinstead of TiW. Because the seed layer, a Al(Si)/TiW/Cu stack in thepreferred embodiment, is not required to have significantcurrent-carrying capacity, it can be made very thin. Hence themanufacturing cost of depositing the seed layer is low. The metal layercomprises a Al(Si)/TiW/Cu stack, where the aluminum provides ohmiccontact and back surface reflectance, TiW acts as the barrier layer, andCu acts as the plating base. Alternatively, chromium (Cr) can be used asthe barrier layer instead of TiW. The metal semiconductor contact can beannealed in a forming gas atmosphere, preferably at 400° C.Alternatively, the contact anneal step can be eliminated.

[0019] Next, as shown in FIG. 6, a patterned plating resist 48 isapplied to the seed layer. In the preferred embodiment, the platingresist is directly patterned on the wafer. After application, theplating resist is cured to harden it against the subsequentelectroplating solution. Metal does not plate in areas covered by theplating resist. Alternatively, the barrier layer can be selectivelypatterned and etched before plating to limit plating area.

[0020] In FIG. 7, the thickness of the metal layer in regions withoutplating resist is greatly increased by electroplating or electrolessplating a good electrical conductor to act as low series resistancemetal grid lines 50, 52. In the preferred embodiment, about 20 μm ofcopper are electroplated. A thin capping layer, such as tin or silver ornickel, may be plated after the copper to improve solderability and/orto prevent etching of plated areas during etch back. Preferably about 7μm of tin are electroplated.

[0021] Finally, as shown in FIG. 8, plating resist 48 is stripped andthe metal film is etched to remove the thin seed layer 44 between theplated conductive lines. The etch back chemistries are chosen such thatthey selectively etch the seed metal stack components over the platedmetal capping layer. Alternatively, a small amount of metal on theplated conductive lines may be sacrificed during etchback if a cappinglayer is not used, or if it is not selective to the etchbackchemistries.

[0022] The final structure is shown in perspective view in FIG. 1showing the interdigitated metal contacts 50, 52 to the p+ regions andn+ regions, respectively, of the solar cell.

[0023] The stacked metal contacts in accordance with the inventionprovide good ohmic connection and reflection properties on the back sideof a solar cell. A number of alternative processing steps and structuralelements have been suggested for the preferred embodiment. Thus whilethe invention has been described with reference to specific embodiments,the description is illustrative of the invention and is not to beconstrued as limiting the invention. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

1. In the fabrication of a solar cell in a semiconductor substrate, amethod of fabricating metal contacts on a surface of the solar cellcomprising the steps of: a) forming a first thin metal layer in contactwith the semiconductor substrate, b) forming a thin barrier metal layerover the first metal layer, and c) increasing thickness of the barrierlayer by plating.
 2. The method as defined by claim 1 wherein the firstmetal layer functions as an infrared reflector.
 3. The method as definedby claim 2 wherein the thin barrier layer comprises a refractory metaland a strike metal that serves as a base to initiate plating.
 4. Themethod as defined by claim 3 and further including before step c) thestep of applying a plating resist to limit plating area.
 5. The methodas defined by claim 4 and including after step c) the steps of removingthe plating resist, and removing by etching exposed barrier metal andunderlying first metal.
 6. The method as defined by claim 5 wherein thefirst metal layer comprises aluminum, the refractory metal comprisestitanium tungsten, and the plating metal comprises copper.
 7. The methodas defined by claim 2 wherein the first metal comprises aluminum and thebarrier layer comprises a metal chosen from chromium and titaniumtungsten.
 8. The method as defined by claim 7 wherein the barrier layercomprises a plating metal which serves as a strike layer to initiateplating.
 9. The method as defined by claim 8 and further includingbefore step c) the step of applying a plating resist to limit platingarea.
 10. The method as defined by claim 9 and including after step c)the steps of removing the plating resist, and removing by etchingexposed barrier metal and underlying base metal.
 11. The method asdefined by claim 1 and further including before step c) the step ofselectively etching the exposed barrier metal layer to limit platingarea.
 12. The method as defined by claim 1 wherein the barrier layercomprises a metal which functions as a diffusion barrier and as a strikelayer to initiate plating.
 13. In the fabrication of a solar cell, amethod of fabricating metal contacts to doped regions in a surface of asemiconductor wafer comprising the steps of: a) forming a silicon oxidelayer over the surface of the semiconductor wafer, b) forming a seedmetal layer over the silicon oxide layer and contacting the dopedregions through the oxide layer, c) forming a plating resist over theseed metal layer to thereby define the geometry of metal contacts, d)plating an electrically conductive layer over the seed layer, e)stripping the plating resist, and f) selectively etching the seed metallayer that was under the plating resist.
 14. The method as defined byclaim 13, wherein the seed metal layer comprises a first layer ofaluminum which functions as a reflector.
 15. The method as defined byclaim 14, wherein the seed metal layer includes a diffusion barrierlayer of titanium tungsten.
 16. The method as defined by claim 15,wherein the seed metal layer includes a conductive layer of copper whichfunctions as a strike layer to initiate plating.
 17. The method asdefined by claim 13 wherein step a) includes etching holes through thesilicon oxide to permit the seed metal layer to contact the dopedregions in step b).
 18. The method as defined by claim 13 wherein stepc) includes defining contacts and a grid pattern.
 19. The method asdefined by claim 13 and further including after step d) the step ofapplying a cap layer over the electrically conductive layer.
 20. Themethod as defined by claim 19 wherein the cap layer comprises platedtin.
 21. The method as defined by claim 19 wherein the cap layercomprises an electroless plated metal.
 22. In the fabrication of a solarcell, a method of fabricating a contact on a surface of a semiconductorwafer comprising the steps of: a) forming a thin base metal layer on thesurface, b) forming a thin barrier metal layer, including a strikelayer, over the base metal layer, and c) forming a conductive layer overthe barrier layer by plating metal on the strike layer.
 23. The methodas defined by claim 22, wherein the base metal layer comprises aluminum,the barrier layer comprises titanium tungsten, and the strike layer andthe conductive layer comprise copper.
 24. The method as defined by claim22 wherein the base metal layer comprises aluminum, and the barriermetal layer comprises chromium.
 25. In a semiconductor solar cell, ametal contact structure comprising: a base metal in contact with asemiconductor substrate of the solar cell, and a barrier metal overlyingthe base metal, the barrier metal including a strike metal layer and ametal plated on the strike layer.
 26. The metal contact structure asdefined by claim 25 wherein the base metal functions as an infraredreflector.
 27. The metal contact structure as defined by claim 26wherein the barrier metal includes a refractory metal and a platingmetal on the refractory metal.
 28. The metal contact structures asdefined by claim 27 wherein the base metal comprises aluminum, thebarrier metal comprises titanium tungsten.
 29. The metal contactstructure as defined by claim 28 wherein the plating metal and strikemetal comprise copper.
 30. The metal contact structure as defined byclaim 27 wherein the base metal comprises aluminum, and the barriermetal comprises chromium.
 31. The metal contact structure as defined byclaim 30 wherein the plating metal comprises copper.
 32. The method asdefined by claim 17 wherein step a) includes applying a chemical pasteon the silicon oxide layer where holes through the silicon oxide are tobe etched.
 33. The method as defined by claim 32 wherein the chemicalpaste is applied by printing.